It is known that the nano-scaled materials exhibit extraordinary electrical, optical, magnetic, chemical and biological properties, which cannot be achieved by micro-scaled or bulk counterparts. The development of nano-scaled materials has been intensively pursued in order to utilize such properties for various technical applications including biomedical and nano-bio applications.
Ti and Ti alloys are corrosion resistant, light, yet sufficiently strong for load-bearing, and are machinable. They are one of the few biocompatible metals which osseo-integrate (direct chemical or physical bonding with adjacent bone surface without forming a fibrous tissue interface layer). For these reasons, they have been used successfully as orthopaedic (orthopedic) and dental implants. See Handbook of biomaterial properties, ed. J. Black and G. Hasting, London; Chapman & Hall, 1998; Ratner et al., Biomaterials Science, San Diego, Calif., Academic press, 1996.
The bioactivity of Ti, such as the relatively easy formation of hydroxyapatite type bone mineral on Ti is primarily caused by the occurrence of Ti oxide on the surface of Ti and its alloys. Among the various crystal structures of Ti oxide, the anatase phase is known to be better than the rutile and other phases. See, e.g., Uchida (2003) J. Biomedical Materials Res. 64:164-170. Surface treatments such as roughening by sand blasting, formation of anatase phase TiO2, hydroxyapatite coating, or other chemical treatment have been utilized to further improve the bioactivity of Ti surface and enhance bone growth.
While the fabrication of vertically aligned TiO2 nanotubes on Ti substrate was demonstrated by anodization process, an investigation of such titanium oxide nanotubes for bone growth or other bio application has not been attempted. An investigation of such titanium oxide nanotubes for bone growth type bio application has only recently been reported, showing a significantly enhanced bone growth on TiO2 nanotube array structure. See, e.g., Oh (2005) “Growth of Nano-scale Hydroxyapatite Using Chemically Treated Titanium Oxide Nanotubes”, Biomaterials 26:4938-4943. Patients who go through Ti implant operations for repair of hip joints, broken bones, or dental implants often have to wait for many months of slow bone growth recovery before they are cured enough to get off the confinement on a bed or crutches and have a normal life. Accelerated bone growth would thus be very beneficial for such patients.
The structure of the anodized TiO2 nanotube array, such as the diameter, spacing and height of nanotubes, is not always easy to control during the electrochemical anodization process of pore formation. For example, the largest reported diameter of TiO2 nanotubes is less than approximately (about) 100 to 150 nm. While a portion of filopodia, the thin branches of growing cells, can get into such a small pores and enhance cell adhesion/growth, the approximately 100 nm regime of dimension is too small to accommodate the main part of typical osteoblast and many other cells as these have a much larger dimension of micrometers. In addition, the desired insertion of biological agents such as biomolecular growth factors, cytokines, collagens, antibiotics, antibodies, drug molecules, small molecules, inorganic nanoparticles, etc. within the pores for further accelerated cell/bone growth or for medical therapeutics can be facilitated if the inner diameter of the pores are made somewhat larger. Therefore, an ability to artificially design and construct a biocompatible nanostructure, e.g., with a specific desired nanotube diameter, nanopore dimension and spacing, is desirable for further controlled and accelerated growth of bones and cells. For orthopaedic and dental applications, a dual structure of larger dimension pores, which in one aspect can be of re-entrant shape, in combination of nanostructured surface would be desirable to have both accelerated cell/bone growth and physically locked-in bone configuration in the re-entrant large pores for improved mechanical durability on tensile or shear strain. Furthermore, if such a biocompatible nanostructure can be made to easily accommodate biological agent storage in the nano/micro pores to enhance multifunctional roles to additionally accelerate bone and cell growth, its practical usefulness can be much enhanced for various biomedical applications.
Coating of bioactive materials such as hydroxyapatite and calcium phosphate on Ti surface is a commonly used technique to make the Ti surface more bioactive for bone growth purposes. See, e.g., Shirkhanzadeh (1991) J. Materials Science Letters volume 10; de Groot (1987) J. Biomedical Materials Res. 21:1375-1381; Cotell (1992) J. of Applied Biomaterials 8:87-92. However, the fatal drawback of these currently available coating techniques is that such a flat and continuous coatings tend to fail by fracture or de-lamination at the interface between the implant and the coating as an adhesion failure, or at the interface between the coating and the bone, or at both boundary interfaces. Thick film coatings tends to introduce more interface stresses at the substrate-coating interface, especially in view of the lack of strong chemical bonding or the absence of common elements shared by the substrate (e.g., Ti implant) and the coating material. See, e.g., Yang (1997) J. Biomedical Materials Res. 36:39-48. It would thus be desirable if the interface is bonded with an improved and integrated structure, for example, with a locked-in configuration with a much increased adhesion area, and as a discrete, less continuous layer to minimize interface stress and de-lamination.
An additional, worthy consideration of bone growth/repair implants is the ability of the implants to withstand a tensile or shear stress, which tends to break off the interface bonding between the implant and the bone that is allowed to grow on the implant surface. It would thus be desirable if the surface geometry of the implant is improved so that not only nanoscale interfacial adhesions occur, but microscale and macroscale lock-in structure is provided to guard against slippage of the implant on tensile stress or breakage of the bond on shear stress.
Accelerated cell growth is also desirable not only for bones but also for a variety of cells including liver cells, kidney cells, blood vessel cells, skin cells, periodontal cells, stem cells, and so forth. Liver in human body is the largest gland and a dynamic organ which serves several important functions, working closely with many fundamental biological systems and bio-processes in the body. The liver is like the main chemical factory and food storehouse in human body, as it helps the body digest food and help purify the blood of the poisons and wastes. The complex functions associated with the liver include; (a) The regulation of blood glucose level, lipids and amino acids, (b) The production and secretion of bile, red blood cells, blood proteins (such as albumin, globulin, fibrinogen), cholesterol, and glucose, (c) The purification of blood by removing toxins, wastes, unnecessary hormones, and hemoglobin molecules, (d) The storage of blood, vitamins and minerals.
The parenchymal cells known as hepatocytes are the major cells populated in the liver. In additions, several other cells such as endothelial cells, adipocytes, fibroblastic cells and Kupffer cells are also included in the liver.
A significant portion of the human population (e.g., about one in ten people) has been afflicted with liver diseases such as hepatitis, liver cancer, and acute or chronic liver failure. Although liver transplantation is an optional treatment method, there is a very limited supply of donor organs, and the medical and associated costs for the transplant procedure and post-operation immunosuppressive drug therapy are considerable.
Many research investigations related to liver cell culture in vitro have been conducted to figure out the problem often caused by long-term culture of liver cells. Cultured liver cells can be useful for hepatocytes transplantation, implantable constructs and bioreactor production. The primary cultures of rat hepatocytes have been extensively used to research the effects of potential toxins on enzyme leakage, metabolism, and cellular membranes. See, e.g., Grisham (1979) International Review of Experimental Pathology 20:123-210; Acosta (1981) Biochemical Pharmacology 30:3225-3230. However, there are a number of known drawbacks about long-term liver cell culture as some loss of liver function is frequently observed. So far, there has been no successful means of proliferating healthy liver parenchymal cells.
In vitro culture of adult hepatocytes does not show prolonged ability to produce albumin and display cytochrome P-450 enzyme activity. In suspension culture, the viability of hepatocytes and their cytochrome P-450 enzyme activity declines gradually as a function of incubation time. In addition, cell division usually is limited to the first 24-48 hr of culture after which the cell division is no longer significant. See, e.g., Sirica (1980) Pharmacology Review 31:205-228; Clayton (1983) Molecular and Cellular Biology 3:1552-1561; Chapman (1973) J. Cell Biology 59:735-747. In a two-dimensional culture system, the viability of adult hepatocytes adhered to the culture plate show somewhat longer activity periods than other culture systems, but the functionality of hepatocytes decreased rapidly. See, e.g., Deschenes (1980) In Vitro 16:722-730.
To improve hepatocyte growth and prolong liver-specific functions in vitro, various kinds of matrices have been studied, such as type I and IV collagen substrates, homogenized liver biomatrix (see, e.g., Reid (1980) Ann. N.Y. Acad. Sci. 349:70-76), sandwich-shaped collagen substrate composed of two layers of type I collagen, and fibronectin coated plates. See, e.g., Michalopoulos (1975) Experimental Cell Res. 94:70-78, Bissell (1987) J. Clinical Investigation 79:801-812; Dunn (1989) FASEB J. 3:174-177; Deschenes (1980) In Vitro 16:722-730. Even though many of these experimental approaches have demonstrated an extended viability of hepatocyte and the stability of liver specific function under in vivo environment, they are still not satisfactory enough for practical applications.
An alternative way, which allows liver cells to possess some long-term viability and liver-specific functionality, utilized co-culturing liver parenchymal cells with a diversity of structurally supportive, non-parenchymal stromal cells or non-hepatic stromal cells. See, e.g., Allen (2005) Toxicological Sciences 84:110-119; Bhatia (1998) Biotechnology Progress 14:378-387. Adult hepatocytes co-cultured with endothelial cells of the same species showed good maintenance of liver-specific functions for several weeks in vitro, even though they did not show significant expansion in cell population. See articles by Guguen-Guilluozo (1983) Experimental Cell Res. 143:47-54; Begue (1983) Biochemical Pharmacology 32:1643-1646. In addition, rat hepatocytes which were co-cultured with human fibroblasts and endothelial cells were reported to exhibit stable cytochrome P-450 activity for more than 10 days. See, e.g., Kuri-Harcuch and Mendoza-Figueroa (1989) Differentiation 41:148-157; Begue (1983) Biochemical Pharmacology 32:1643-1646. Therefore, mixed hepatocyte co-culture systems with non-liver derived cells may provide microbiological environments similar to those in vivo by optimizing cell-cell interactions. However, there are still problems about the nature of non-liver derived cells. The viability and functional activities of co-cultured hepatic primary cell can be prolonged in vitro, but primary cell proliferation is limited or absent in these system, which is a critical flaw. Even though several reports indicate that non-parenchymal liver cells may express functions similar to hepatocytes, the nature of non-liver derived cells co-cultured with liver primary cells has not been established unequivocally. See, e.g., Grisham (1980) Annals of the NY Acad. Sci. 349:128-137. It is therefore highly desirable to develop culture methods and culture devices that can allow artificial in vitro (or in vivo) growth of healthy, fully functional and long-lasting liver cells that can be transplanted to the patients in need of liver cells.
There is also a critical need for an artificial liver device that can remove toxins and improve immediate and long-term survival of patients suffering from liver disease. An artificial liver device can be useful as a temporary artificial liver for patients awaiting a liver transplant, and also provide support for post-transplantation patients until the grafted liver functions adequately to sustain the patient. One of the major roadblocks to the development of an effective artificial liver device is the lack of a satisfactory liver cell line that can provide the functions of a liver.
Yet another benefit of being able to culture healthy liver cells is to meet the demands for supply of the cells for toxicity testing of enormous numbers of new or experimental drugs, chemicals, and therapeutics being developed in the pharmaceutical and chemical industry. With the unique toxin-filtering capability of liver cells, any toxicity of a new drug can be manifested first by the reaction of the liver cells. An array of liver cells can thus be utilized as a fast testing/screening vehicle to basically simultaneously evaluate the potential toxicity of many new drugs and compounds.
Two-dimensional and three-dimensionally cultured cells are useful not only for liver cell related applications, but for producing a number of other cells in a healthy and accelerated manner. There are needs to supply or implant various types of cells including bone cells, liver cells, kidney cells, blood vessel cells, skin cells, periodontal cells, stem cells, and other human or animal organ cells.
A fast growth and supply of cells especially rare cells, such as stem cell enrichment, can be crucial for many potential therapeutic applications as well as for enhancing the speed of advances in stem cell science and technology. In addition, fast detection and diagnosis of disease cells or possible bio-terror cells (such as epidemic diseases, anthrax or SARS) from a very small or trace quantity of available cells can be accomplished if the cell growth speed can be accelerated.